The present invention relates to a system for processing a digital picture data signal where the picture data signal for a screen of a particular size is converted to a picture data signal appropriate for a screen of another size.
An ordinary NTSC television receiver has a screen, the aspect ratio of which is 4:3. On the other hand, the screen of a cinema-size television receiver has the aspect ratio of 16:9. When picture data signals for the cinema-size television receiver are applied to the NTSC television receiver to show a picture on the screen thereof, there is formed non-displaying areas at upper and lower portions of the screen. Namely, a so-called letter box display is carried out. When the displaying picture is expanded in the vertical direction, the screen is efficiently used, thereby enabling to show a larger picture.
More particularly, such an expansion process of the picture is carried out as follows. An input image data is written in a frame memory in synchronism with an input clock signal, and read out from the frame memory in synchronism with an output clock signal, one line at a time. The read out image data is filtered by a vertical filter so that the number of scanning lines is increased, for example, by 4/3.
However, during the expansion process, since the processing rate at the input side, that is the signal source side, differs from that at the output side. Thus, it is necessary to provide a frame memory as a buffer so that an asynchronous components of the signal can be absorbed. As a result, the size of the circuit becomes large, thereby increasing the manufacturing cost thereof. Hence, a large scale integration is difficult to realize.